aklog: Fix improper use of readlink

readlink doesn't NUL terminate its return string, so it is up to
us to do so.

Caught by coverity (#985739)

Change-Id: Ifb858d628845bd963928e25834e540bbb3a187c8
Reviewed-on: http://gerrit.openafs.org/9347
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>